Oil shales of the United States are characterized as sedimentary rocks which contain a dispersed, organic constituent known as kerogen. In some cases, other organic materials, such as coal and bitumen, can be interspersed with kerogen in the shale, contributing to a complex organic content. The Western oil shales of the United States, in Colorado, Utah, and Wyoming, are frequently known as the Eocene shales of the Green River formation. Oil shales of the eastern United States, principally in Indiana, Tennessee, Kentucky, and Ohio, are frequently referred to as Devonian, Mississippian, and Ordovician shales, which refer to their period or era of deposition. The Eastern shales originate principally in formations referred to as Chattanooga shale, New Albany shale, Antrim shale, and Appalachian shale.
Retorting of the various qualities of shale causes pyrolytic decomposition of the organic matter, which degrades into fixed carbon, noncondensable-combustible gases, and condensable oils referred to as crude shale oil. Standard analytical methods of determining oil assays of oil shales involve Fisher retorting tests, which provide preliminary data concerning oil and gas yields. Analysis of spent shale provides analytical information with respect to residual fixed carbon in retorted shales.
The western Colorado shales have a high yield of oil from pyrolytic decomposition (retorting) of the contained organic matter. Generally, 65 to 85 percent of the organics (principally carbon and hydrogen as hydrocarbon) in Western shales convert to oil and gases and the residual organic matter converts to fixed carbon within the spent shale. It is common to have Western shales containing about 12 to 15 percent organics to yield about 30 gallons of oil per ton of shale and have 2 to 4 percent carbon residual in the spent shale, which indicates a relatively high yield for the recovery of organics as oil.
The Eastern shales, on the other hand, can have a comparable content of organics in the raw shale, but when they are retorted, they have lower yields of oil and gas, and have a relatively high content of fixed carbon residuals in the shale. This factor is especially prominent when the eastern shales contain coal and other carboniferous matter with the kerogen. An example of retorting Eastern shales containing approximately 12 to 13 percent organics shows an oil yield of about 10 gallons of oil per ton of shale, with only about 30 percent of the organics converted into oil. Consequently, spent shales have a fixed carbon content on the order of 6 to 10 percent.
The reasons for the varying oil yields from the organics are not precisely known. In some cases, the carbon:hydrogen ratios of the original organics can be used as guides concerning oil yields. For instance, Western oil shales have a carbon:hydrogen ratio on the order of 6.5 to 7.5, whereas, the Eastern shales have a carbon:hydrogen ratio range from about 6.8 to 11.0. Normal coal, for instance, has a carbon:hydrogen ratio of 10. When coal is retorted, very high percentages of fixed carbon-coke residue are evident.